drm/i915: Remove vblank wait from hsw_enable_ips, v2.

intel_post_plane_update did an extra vblank wait that's no longer needed when enabling ips.

Changes since v1:
- Add comment explaining why vblank wait is performed. (Paulo)

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/56F29B28.5070804@linux.intel.com